diff --git a/include/boost/system/error_code.hpp b/include/boost/system/error_code.hpp
index eb22f8e..8c433d3 100644
--- a/include/boost/system/error_code.hpp
+++ b/include/boost/system/error_code.hpp
@@ -452,14 +452,18 @@ inline const error_category & generic_category() BOOST_SYSTEM_NOEXCEPT
 namespace detail
 {
 
-#if defined(BOOST_SYSTEM_SOURCE)
+#if defined(BOOST_SYSTEM_SOURCE) || defined(_MSC_VER)
 
 // clang++ requires a strictly matching declaration
+// MSVC requires __declspec(dllimport)
+
 BOOST_SYSTEM_DECL extern system_error_category system_category_instance;
 BOOST_SYSTEM_DECL extern generic_error_category generic_category_instance;
 
 #else
 
+// g++ constexpr requires absence of __declspec(dllimport)
+
 extern system_error_category system_category_instance;
 extern generic_error_category generic_category_instance;